We believe we have discovered a novel explanation for the mechanism by which serotonin (5- HT) produces pulmonary artery smooth muscle cell (SMC) proliferation and migration. This is important since these SMC functions participate in pulmonary hypertension (PH) and the 5- HT/5-HT transporter (SERT) system has been associated with clinical and experimental PH. From preliminary data the biochemical process of posttranslational modification of SMC protein through transamidation (serotonylation) leads to SMC proliferation and migration. Fibronectin (FN), an important extracellular matrix protein that has been associated with PH, is a major protein undergoing serotonylation. This enhanced serotonylation of SMC protein, occurring through the tissue enzyme transglutaminase (TGase), is influenced by the PDGF receptor and possibly other cell surface receptors that have been associated with PH. Furthermore, as an in vivo corollary enhanced serotonylation of FN occurs in lung tissue of mice and rats developing PH secondary to exposure to hypoxia and in rats given monocrotaline, and we have identified marked elevation of serotonylated FN in lungs and sera of some patients with PAH. We propose in this application to explore further the roles that lung tissue SERT, TGase and protein serotonylation may have in stimulation of SMC proliferation and migration and development of PH with a variety of cell culture, transfection, siRNA, immunoprecipitation and immunoblotting techniques and with the use of normal and transgenic rodent model experiments. Specifically, we will 1) better define the roles of SERT and TGase in SMC proliferation, migration and cell signaling produced by 5-HT; 2) determine the roles of serotonylation of FN and related integrins in 5-HT induced SMC proliferation and migration; 3) assess modification of SMC protein/FN serotonylation by hypoxia, PDGF/PDGFR and BMP/BMPR, known participants in PH; and 4) examine the association of lung and pulmonary artery protein/FN serotonylation in rodents developing PH secondary to hypoxia or following administration of monocrotaline, in SERT KO and knock in transgenic mice, in SERT KO rats and in mice deficient in TGase. The SERT knock-in animals that show overactivity of SERT are from Dr. Randy Blakely, an internationally recognized expert in SERT, at Vanderbilt University. Dr. Stephanie Watts from Michigan State University, another expert in SERT, will collaborate in studies related to SERT KO rats and their isolated pulmonary artery SMC's. We hope to better define any role of SERT, TGase activity and protein serotonylation in the development of PH and to consider possible new interventional strategies through these pathways that might be used for treating this disease.